User mobility is central to mobile wireless communications systems. Consequently, a wireless system user frequently is in motion during an active call. In such cases, the user may pass out of range of the base station with which the user's station set (also commonly referred to as a user terminal, mobile terminal, or handset) is presently communicating. Many wireless systems provide the ability for a call to undergo a "handoff," where the system moves the call from the old base station that is presently serving the call to a new base station closer to the user's present position. It is not unusual for the parties to the call to hear an audible glitch as the call is handed off, due to poor coordination of the timing of the handoff. This glitch is usually caused by lost or garbled voice samples during the switch-over as the old base station stops transmitting and receiving and the new base station begins.
Traditional approaches to solving this problem often attempt to eliminate the glitch within the wireless system by temporarily having both the new and the old base stations receive the call traffic from the mobile terminal and conferencing the traffic received from the two base stations during the handoff. This may reduce the glitch heard by the other party to the call but does not eliminate the glitch heard by the mobile wireless user whose motion caused the handoff to occur. Other approaches place undesirable constraints on system operation or user mobility.
Newer digital cellular offerings smooth the handoff process by a mechanism known as "soft" handoff, where traffic passed to and from the mobile terminal is replicated by both the new and the old base stations during the handoff. By replicating the traffic, voice samples and signaling messages are less likely to be lost, and glitches are reduced. However, replicating the traffic and handling the replicas within the wireless system is complex and costly, requiring expensive hardware for traffic duplication and selection and duplicate-traffic synchronization. Moreover, it is only applicable to certain technologies - i.e., CDMA.